Pacing is most closely associated with film and theater, but does have an impact on all forms that have a serial component. It is the speed at which the work or even a part of that work proceeds. This pacing usually has a direct connection with the desired energy of that work.
In music, the pacing best correlates to tempo. That tempo is often indicated by the composer, but is interpreted by either the players, or the conductor (if there is one).
In theater, pacing is often controlled through dialogue and physical movement.
In film, those are important but, editing and cinematography as well as the soundtrack (music) will also come into play. Most action oriented works have a generally faster pace. Most dramas have a slower pace. However, bear in mind that there will often be fluctuation of the pacing throughout the film so as to not make it monotonous. Several things contribute to pacing in a film including: the sound track, movement of the objects within the scene, camera movement, transitions, and the lengths of segments.